In the manufacture of printed paper, paperboard and other printed web products, words and designs in various colors are applied to a surface of a web or sheet using a rotating transfer roll. In a conventional printing process known as the gravure process, the transfer roll is a cylinder having cells provided about its peripheral surface. The cells are small, shallow depressions engraved in the surface of the roll which retain ink or other printing fluid when passed through a fluid bath or a fluid applicator chamber. The cylinder is provided between the fluid chamber and an impression roll and is positioned for rotational contact with the impression roll. The cylinder and the impression roll rotate in opposite directions, with the web being pressed therebetween. As the cylinder rotates, printing fluid from the fluid chamber is retained in the cells of the cylinder. Prior to the press nip, a doctor blade is positioned for contacting the peripheral surface of the transfer roll. The edge of the doctor blade levels and removes excess printing fluid from the surface of the cylinder. At the nip,. printing fluid is released from the cells and transferred to the surface of the web.
In another printing process known as flexographic printing, a plate cylinder having raised rubber regions extending from its peripheral surface is provided between the transfer, or anilox, roll and an impression roll. The anilox and impression rolls rotate in the same direction, while the plate cylinder rotates in an opposite direction and contacts the peripheral surfaces of both rolls. The web is pressed between the impression roll and the plate cylinder. Cells provided on the outer surface of the rotating anilox roll retain printing fluid from the fluid chamber. A doctor blade positioned before the contact point between the anilox roll and the plate cylinder meters the fluid onto the surface of the anilox roll. The fluid is transferred from the surface of the anilox roll to the raised regions of the plate cylinder, which in turn transfers the fluid to the web at the press nip.
Doctor blade systems in existing printing press machines generally include two or more doctor blades which contact the peripheral surface of a rotating roll. At least one, and typically all the blades, are reverse angle blades, or blades positioned such that the roll surface rotates toward the doctoring edge. A typical doctor blade assembly having a reverse angle blade in combination with a trailing angle blade (i.e., blade positioned such that the roll surface rotates away from the blade) is disclosed in U.S. Pat. No. 4,945,832.
According to the apparatus disclosed in U.S. Pat. No. 4,821,672, a doctor blade assembly includes a pair of blades disposed on opposite sides of an applicator ink cavity. The blades converge as they extend toward the transfer roll. A pivot shaft and handle assembly operate to move the blades and applicator into and out of engagement with the transfer roll. Seal assemblies, including end plates resiliently bearing on the end surfaces of the transfer roll and seal members secured to the end plate and disposed to engage the endmost portion of the circumferential roll surface, are provided for preventing seepage of ink from the cavity. The enclosed wet angle defined between the upper reverse angle doctor blade and the lower trailing angle doctor blade is usually small, generally in the range of ninety degrees or less.
According to the apparatus disclosed in U.S. Pat. No. 5,239,925, an ink distribution assembly for flexographic or gravure printing presses provides for limited exposure of transfer rolls to the atmosphere. To function properly, the assembly must include three or more doctor blades defining two or more adjacent reservoirs. The doctor blade immediately following the press nip extends at a trailing angle to the transfer roll but does not contact the transfer roll. The intermediate blades and the lower reverse angle blade are in constant contact with the transfer roll surface.
According to the apparatus disclosed in U.S. Pat. No. 2,151,968, an inking mechanism for a rotogravure press includes an ink channel defined by a bottom plate, a trailing angle doctor blade and a sealing blade. The sealing blade is formed of very thin steel or of a non-metallic composition and has a curved lower end which extends along the peripheral surface of the printing cylinder. The ends of the channel are not sealed to the printing cylinder. Rather, raised bands provided along the bottom plate and ink guards provided at the ends of the printing roll function to limit discharge of ink through the ends of the ink channel. The surface of the printing roll is exposed to the atmosphere for a majority of its rotation.
It has been found that acceptable printing quality depends on uniform coating of the transfer roll. The term "transfer roll" as hereinafter used includes both anilox rolls for flexographic presses and print cylinders of gravure printing presses unless otherwise stated. Printing fluid tends to dry in the cells of the transfer roll when the surface of the transfer roll is exposed to atmospheric conditions. The dried printing fluid clogs the cells and prevents the cells from retaining adequate levels of transferrable fluid. Streaking and other undesired printing phenomena result. Cleaning or replacing the rolls with clogged cells is time consuming and costly.
It has been further found that the ink applicator cavity for printing presses must be sealed for effectively recycling the printing fluid and for reducing the escape of solvent vapors emanating from the printing fluid into the work environment. Exposing printing fluid on the print cylinder or in the applicator cavity to atmospheric conditions causes the fluid to dry or to thicken. Further, seepage of the ink from the cavity and excess ink flung from rapidly rotating rollers necessitate inconvenient and often troublesome cleanup. It has been further found that reverse angle blades of doctor blade systems cause increased wear to the transfer roll surface.
It would be desirable therefore to provide a method of and an apparatus for minimizing exposure of the print cylinder in a gravure press to atmospheric conditions to eliminate the aforementioned problem of escape of solvent vapors to the work environment.